We report on the generation of a narrow divergence (θ ≈ 2.5 mrad), multi-MeV (EMAX = 18 MeV) and ultra-high brilliance (≈ 2 × 10 19 photons s −1 mm −2 mrad −2 0.1% BW) γ-ray beam from the scattering of an ultra-relativistic laser-wakefield accelerated electron beam in the field of a relativistically intense laser (dimensionless amplitude a0 ≈ 2). The spectrum of the generated γ-ray beam is measured, with MeV resolution, seamlessly from 6 MeV to 18 MeV, giving clear evidence of the onset of non-linear Thomson scattering. The photon source has the highest brilliance in the multi-MeV regime ever reported in the literature.The generation of high-quality Multi-MeV γ-ray beams is an active field of research due to the central role of these beams not only in fundamental research [1], but also in extremely important practical applications, which include cancer radiotherapy [2,3], active interrogation of materials [4], and radiography of dense objects [5]. As an example, Giant Dipole Resonances of most heavy nuclei occur in an energy range of [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30], exciting photofission of the nucleus.Different mechanisms have been proposed to generate γ-ray beams with these properties, including bremsstrahlung emission, synchrotron emission, and Compton scattering. Bremsstrahlung sources are routinely used for medical applications, and exploit electron beams accelerated by linear accelerators (LINAC) [7]. Laser-driven bremsstrahlung sources, whereby the electron beam is generated via laser-wakefield acceleration (LWFA) [8] have also been recently reported [5,9,10]. However, the relatively broad divergence and source size limit the maximum brightness achievable with this technique and a more promising physical mechanism in this respect has been identified in Compton scattering. Laserdriven electron beams with energy per particle of the order of the GeV are now routinely available in the laboratory [8], allowing for the possibility of all-optical and compact Compton-scattering sources [11,12].Previous investigations of laser-driven Compton scattering have mostly focused on the linear regime, i.e. whenever the dimensionless intensity of the laser pulse is less than 1 (a 0 < 1, whereby a 0 = eE L /(m e ω L c), with E L and ω L being the laser electric field and central frequency, respectively, and m e being the electron rest mass) [13,14] and report on γ-ray energies ranging from a few hundreds of keV [13] up to 3-4 MeV [14]. Three main factors can in principle be modified in order to increase the energy of the generated photons: the electron Lorentz factor (γ e ), the laser photon energy ω L , or the laser intensity a 0 . The mean energy of the generated photons can in fact be estimated as:with f (a 0 ) ≈ 1 for a 0 1 andLiu and collaborators recently reported on an increase in photon energy (up to 8-9 MeV) by frequency converting the scattering laser up to its second harmonic (thus increasing ω L in Eq. 1) [15]. However, using a higher laser frequency for scattering significantly ...

All-atom molecular dynamics simulations with explicit water solvent were used to investigate the microstructure and conformational dynamics of cellulose Iβ microfibrils as a function of microfibril length and cross-sectional size and shape. Cellulose microfibrils quickly develop a right-handed twist, which then remains stable over the entire 10 ns simulation time. The helical angle is independent of microfibril length and inversely proportional to its cross-sectional area, in accord with the expectations of continuum theory for an intrinsic chiral twist that is opposed by torsional shear. These calculations provide-to our knowledge-the first estimates of the shear modulus of a cellulose microfibril from MD simulations. The internal strains caused by this helical twist, propagated indefinitely along the microfibril axis, could be relaxed by periodic regions of amorphous structure along the axis of the cellulose microfibrils.

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